An integrated circuit memory, such as a static random access memory (SRAM), is generally implemented as an array of memory cells in a plurality of rows and columns. An array may be subdivided into blocks of memory cells. The memory cells are addressable through block, row, and column decoders for reading data from the memory cells or writing data into the memory cells. During a read cycle, a word line selects the addressed row of memory cells and a pair of complementary bit lines communicate the data between the addressed row and a sense amplifier. The data exists as a relatively small differential voltage on the pair of complementary bit lines. A sense amplifier detects and amplifies the differential voltage and communicates it to the data output stage of the integrated circuit memory via read global data lines.
BICMOS integrated circuit memories are constructed by including bipolar transistors and complementary metal-oxide-semiconductor (CMOS) transistors on the same integrated circuit. Bipolar transistors are often used as output devices in BICMOS circuits to provide high output current and fast switching speed, while CMOS transistors are used in the logic elements because of their low power operation. Sense amplifiers are among the circuits on a BICMOS integrated circuit memory that includes bipolar transistors in order to achieve faster operating speeds.
Generally, a BICMOS sense amplifier circuit includes a differential amplifier to receive signals from a bit line pair. A common-emitter differential amplifier includes a pair of NPN bipolar transistors having their emitters connected together. A constant current source is applied to the coupled emitters, and a relatively small differential input signal from a selected bit line pair is received at the bases of the pair of bipolar transistors. In response, an output signal is provided at the collector of each of the bipolar transistors. A load may be connected between the collectors of the differential pair and the positive power supply voltage. An emitter-follower input or output stage may also be included to drop the input or output signal level depending on the particular application.
Several problems must be overcome when including bipolar transistors in a sense amplifier. In order to receive the benefit of fast operating speeds, each bipolar transistor in the circuit typically should be operated in its nonsaturation region. If a bipolar transistor falls into saturation, it may take a long time for the bipolar transistor to recover from the saturated state, thus decreasing the operating speed of the integrated circuit memory. Also, if a bipolar transistor is subjected to a large reverse-bias across its base-emitter junction, degradation of the transistor occurs. Over time, the constant application of this large reverse bias may cause the bipolar transistor to fail, resulting in a failure of the entire integrated circuit. However, a small reverse bias is acceptable, and may even be desirable in some applications.
In order to reduce the area on the integrated circuit memory occupied by the sense amplifiers, sense amplifiers are sometimes shared by adjacent blocks of memory cells, thus reducing the number of sense amplifiers in the integrated circuit memory. Additionally, it is often desirable in an integrated circuit memory for the outputs of a plurality of sense amplifiers to be wired-OR connected to the read global data line pairs to obtain the logic OR function. In order to be wired-OR connected, the outputs should be electronically deselectable from the read global data line pairs. To reduce power consumption of the sense amplifier when it is deselected, it is advantageous to switch off the sense amplifier as well. However, the resolution of one problem often creates other problems that must also be addressed, further complicating the sense amplifier.
It is a common practice to use a MOSFET transistor as an active load on the collector of each bipolar transistor in the differential amplifier. To reduce power consumption when the sense amplifier is deselected, the load transistors and the current source are switched off so that they are nonconductive, thus preventing any current flow through the sense amplifier. When the load transistors are switched off, the voltage drop across them may cause the bipolar transistors to drop in the saturation region, thus increasing the time it takes to switch the bipolar transistors on again. P-channel transistors are sensitive to the power supply voltage, temperature, process variations, and to the current source. This may cause the drain-source voltage of the P-channel transistor to vary, thus decreasing the noise margin, or causing the bipolar transistors to operate in the saturation region.
If the collector of each bipolar transistor is coupled to the positive power supply voltage terminal, the transistor is prevented from falling into saturation, but then an emitter-follower output transistor of the sense amplifier, when deselected, may be prevented from being wired-OR connected to the read global data line pair with other sense amplifiers. This is because noise, or a voltage spike or glitch from the power supply and through one of the differential transistors may cause the emitter-follower output transistors to become conductive. If one of the emitter-follower output transistors inadvertently becomes conductive when it is wired-OR connected to the read global data line pair, the data on the global data line pair becomes unreliable.
If the current source of the differential amplifier is switched off to prevent current flow when the sense amplifier is deselected, the common emitters may become "floating". If the emitters are floating, there is a potential reverse-bias problem of the differential pair of transistors. One solution to the problem of floating emitters is to use two current sources. One current source for normal operation, and another, smaller current source to prevent the emitters from floating when the sense amplifier is deselected. The problem with using a second current source, although smaller, is that the sense amplifier is still consuming power even though it is deselected. Schottky transistors and diodes have been used to prevent bipolar transistors from operating in the saturation region, but the process steps for fabricating a Schottky transistor are not easily adaptable to the BICMOS fabrication process.